The regeneration of secondary catalyst material has been of particular interest in the chemical industry for many years, since the reuse involves a resource and cost saving.
The field of dehydrogenation catalysts includes the following prior art:
DD 268 631 A1 describes dehydrogenation catalysts which consist of from 50 to 90 parts of waste iron oxides with the composition of from 10 to 40% magnetite, from 50 to 80% goethite and from 10 to 30% lepidocrocite (waste 1), from 1 to 20 parts of waste iron oxides with the composition of from 85 to 95% magnetite (waste 2) and from 5 to 15% wuestite and from 49 to 10 parts of ground spent styrene catalyst. No thermal treatment of the ground spent catalysts is described; it is merely mixed with the other wastes before the further processing. Waste 1 and 2 arises, for example, in the production of pigments for data carriers owing to the high qualitative requirements with regard to the magnetic properties. The dehydrogenation catalyst described achieves a conversion of approx. 40% with a selectivity of approx. 92% in the dehydrogenation of ethylbenzene to styrene.
WO 94/11104 discloses a process for preparing dehydrogenation catalysts comprising iron, potassium and cerium from such spent catalysts by grinding the spent material, if appropriate purifying, restoring the original action by adjusting the composition and restoring the external shape by adding to the ground spent material an effective amount of potassium and such an amount of cerium that the total amount of cerium is higher than the amount originally present. If appropriate, the spent material is calcined in the presence of oxygen before grinding. The process described affords dehydrogenation catalysts which achieve a selectivity of from 94 to 95% with a conversion of 70% in the dehydrogenation of ethylbenzene to styrene.
In other catalytic processes too, for example in the removal of nitrogen oxides from combustion gases by reaction with ammonia at elevated temperature, a regeneration of the spent catalyst material is described (DE 40 06 918 A1), by grinding the spent catalysts to particle sizes of from 5 to 20 μm and adding the resulting powder in the preparation of catalysts with fresh starting material in amounts of up to 80% by weight based on the overall material used before the shaping.
DE 103 05 650 A1 describes the regeneration of mixed oxide catalysts which are used in the ammoxidation to prepare nitriles. The deactivated catalyst is ground if appropriate and calcined under oxygen at from 300 to 900° C.
The prior art describes various catalysts and processes for dehydrogenating and hydrogenating hydrocarbons. The (de)hydrogenation catalysts described are typically supplied in the form of strands, rings, tablets, annular tablets, extrudates, honeycombs or similar moldings. The active composition of the catalysts mentioned comprises predominantly metals selected from the group consisting of iron, alkali compounds, especially potassium, molybdenum, magnesium, calcium, cerium, tungsten, titanium, vanadium, copper, manganese, nickel, zinc, palladium, platinum, cobalt aluminum, tin, silicon, lead, ruthenium, silver, gold, zirconium, rhodium, lanthanum, chromium, cadmium or barium.
The (de)hydrogenation catalysts are prepared in relatively recent prior art and on the industrial scale, however, still by the following processes (see, inter alia, EP-A 1 379 470):    a) The fresh feedstocks in the form of metal oxides, nitrates, carbonates, hydroxides or the like are mixed directly in a mixer, kneader or Mix-Muller. The feedstocks may also be slurried in a spray slurry and processed to a spray powder in a spray dryer. The extrudable mass is subsequently extruded, dried and calcined.    b) The fresh feedstocks are obtained via precipitation reactions, processed to a spray powder in a spray dryer, calcined and reshaped, or first reshaped and then calcined.
When a catalyst, after a running time of typically from two to five years of operation in an industrial plant, for example an isoprene, butadiene or styrene plant, is deinstalled, the catalyst has experienced a number of changes. The deinstalled catalyst generally has iron oxide in a reduced form, i.e. as magnetite. Some of the deinstalled catalyst has generally been depleted of potassium compounds, while an enrichment of potassium compounds may also be present in the form of separate deposits between the catalyst strands in other regions, especially in the interior of the catalyst molding. The potassium is typically present in the form of potassium carbonate or potassium hydrogencarbonate. The deinstalled catalysts generally have virtually no organic hydrocarbons or coke deposits whatsoever, i.e. carbon which is not present as carbonate or hydrogencarbonate. The cerium crystal size of the deinstalled catalysts is from about 40 to 60 nm.
The catalyst moldings have often been damaged by the mechanical stresses in the course of installation, operation and deinstallation. Moreover, the deinstalled catalysts may comprise a high fraction of dust or fragments.
Owing to the changes detailed in the secondary catalyst material and the general difficulties in the processing of secondary catalyst material, especially in the course of extrusion, predominantly catalysts which have been prepared from fresh feedstocks have been used to date in industrial (de)hydrogenation processes.
In view of high raw material prices and rising demands on the sustainability of economic activity, processes for using secondary raw materials are increasingly in the focus of chemical research. Moreover, increasingly higher demands are being made on the disposal of spent catalysts from the chemical industry.